The invention relates generally to the field of optical waveguide manufacturing and particularly to the processing of optical fibers to create fiber Bragg gratings.
Fiber Bragg gratings (FBG""s) are portions of optical waveguides, such as optical fibers, which have been processed to reflect and transmit specific wavelengths. The waveguides are typically germanium-doped silica fibers and for the purposes of this description will be referred to as xe2x80x9cfibersxe2x80x9d or xe2x80x9coptical fibers.xe2x80x9d However, it should be understood that these terms are being used in a generic sense to mean any type of optical waveguide.
Producing FBG""s involves exposing the fiber to ultraviolet light, the intensity of which varies between light and dark along the length of the fiber. The light and dark bands of exposure are placed along the fiber with spacing comparable to the wavelength of light to be reflected by the fiber in operation. The ultraviolet light induces changes in the index of refraction of the fiber, producing an index grating along the length of the fiber.
A light source used for exposure of a fiber to make FBG""s must provide light within specific ranges of wavelengths in the ultraviolet portion of the spectrum. A typical fiber""s primary wavelength range for absorption peaks near 240 nm, and wavelengths differing from the peak by more than about 10 nm are significantly less effective. Even at the peak wavelength, only a small fraction of the laser power is absorbed, so it is highly desirable for the light source to provide light at a wavelength near the absorption peak.
Unfortunately, the current sources of ultraviolet light used for FBG production have various drawbacks. Most current production systems for FBG""s use either KrF excimer lasers emitting at 248 nm, or frequency-doubled argon-ion lasers at 244 nm, to expose the fibers. KrF excimer lasers can produce high average powers, which facilitates processing, but they have serious disadvantages. They require toxic, corrosive gases for operation, have high operating and maintenance costs, and produce short duration (xcx9c50 ns), low repetition rate ( less than 1000 Hz), high peak output power (xcx9c1 MW) ultraviolet pulses. The high peak output powers cause damage to the fibers, weakening them and making them susceptible to fracture. The alternative continuous wave (xe2x80x9ccwxe2x80x9d) argon ion lasers suffer high operating costs and produce weak output powers of one-half watt or less, leading to poor production throughput.
Several other types of lasers, including argon-fluoride and xenon-chloride excimer lasers and flashlamp-pumped lasers, have also been applied to FBG production. However, the argon-fluoride and xenon-chloride excimer lasers suffer from disadvantages similar to those of KrF excimer lasers. Flashlamp-pumped lasers provide some operational improvements compared to excimer lasers, but also generate high peak power pulses which damage the optical fibers.
Other lasers have also been used in laboratory demonstrations of FBG production. Frequency-doubled copper vapor lasers at 255 nm have been used, though their output wavelength is slightly too long to be optimal. Frequency-doubled liquid dye lasers have been tuned to the 240 nm region for FBG fabrication, but such lasers are impractical for large-scale industrial production, since they require very frequent changes of the liquid dye solution to maintain operation.
Solid-state lasers are being increasingly utilized for materials processing applications, due to their reliability and reasonable operating costs. Solid state lasers would be of great interest for FBG production, but heretofore have not been usable because they have not been able to produce the required wavelengths.
According to one embodiment of the present invention, an apparatus for producing a diffraction pattern in an optical fiber includes a solid state laser for producing a fourth harmonic laser beam having a wavelength in the range of approximately 230 to 250 nanometers and means for using the fourth harmonic laser beam to produce a diffraction pattern on an optical fiber.
In one embodiment, the output beam from one of the foregoing embodiments is used to illuminate a proximity mask which causes a diffraction pattern to be imaged onto the fiber. In an alternative embodiment, a diffraction pattern from a phase mask is imaged onto a waveguide, such that a portion of the FBG may have a different period than that of the corresponding groove of the phase mask.
In another embodiment, a holographic technique is employed: a beam splitter splits the output beam into two beams and interference between these two beams is used to create the FBG.
In still another embodiment, the output beam illuminates a phase mask interferometer which produces the FBG. According to several embodiments, the phase mask interferometer has mirrors for reflecting rays diffracted from the phase mask. According to one such embodiment, an actuator controls the lateral movement of at least one mirror. According to one embodiment, the distance between a first mirror and a direct ray is less than the distance between a second mirror and the direct ray. According to another embodiment, one or more mirrors may be rotated by an actuator. In some embodiments, the fiber is kept parallel to grating during FBG production, but in other embodiments the fiber is inclined with respect to the fiber. Moreover, the output beam may illuminate a single portion of the phase mask or may be scanned along the phase mask.
According to yet another embodiment, any of the previously described light sources illuminates a Lloyd mirror interferometer for producing FBG""s.
According to other embodiments, a prism interferometer illuminated by any of the previously described light sources produces FBG""s. According to one such embodiment, the prism interferometer includes a right angle prism. According to another such embodiment, an actuator rotates a prism to tune the Bragg wavelength of an FBG.